The short answer is that they are both "Allied Powers", the United Nations Security Council created after the end of WWII had is permanent members selected to reflect the post-WWII situation. The UNSC had to be comprised of permanent members who had the ability to enforce world peace and thus the natural decision was to include the five most powerful countries at the time which were the major members of the victorious allies. There is no need to arbitrarily end at four members and one could argue that the dominions were major powers although they are normally just rolled together with the UK at this point since their foreign policy was still heavily tied together and influences from London. So with all that being said, there is no reason why they both can't be considered so this argument seems to be founded on an arbitrary decision if nothing else. The normal lens to examine Allied Powers has always been to look at the Big Three or as the P5 members of the security council.

The longer answer is that your concept of what constitutes an Allied Power is based solely on the idea of contribution to "winning" the war. Comparing Nationalist China and France is incredibly difficult since they were effectively fighting in two separate theaters and were involved in the war from the get go. Therefore it is hard to compare strategic impact of any of their actions to one another since they affect two different theaters. It is not the same as examining the presence of the UK or US since they were involved in both thus creating a direct and tangible link. So the only other way to go about this is to quantitatively measure things which again is not an accurate representation. Number of casualties means different things on different fronts, the Western Front had significantly fewer casualties than the Eastern Front but their the impact of the Western Allies and the D-Day landing is not in question with regards to contributing to the end of the war. To put it simply, comparing statistics alone is not a good measure and their context is more or less removed for one another. Its like comparing apples and oranges. They fought in different theaters and contributed to the defeat of a different Axis power. Thus the conclusion most people arrive at is that they are both "Allied Powers" since any comparison is seriously flawed or yields little objective results on an academic level.